In many technical areas, the surface structure of a constructional element or a material is an important quality characteristic.
There are different roughness measurement devices for rapidly collecting the roughness and/or scallop height of surfaces. Typically, during a mechanical scanning, a probe tip is guided over the surface. The result is a height signal recorded over the scanning path, the so-called surface profile.
A distinction is made between so-called runner sensors 1 and reference plane sensors 5, as shown in schematic form in FIGS. 1A and 1B. A runner sensor 1 has a runner 2 that has a large or small radius according to the area of application and that serves as a slide element. The scanner 3 of the runner sensor 1 bears on the surface F to be measured with the runner 2 and collects the surface profile relative to the path of the runner 2 using a probe tip 4 of the scanner 3. During the measurement, this runner 2 follows the macroscopic unevenness of the surface F, hence the waviness and macroscopic shape. On the other hand, the probe tip 4 collects, with its small tip radius, the surface roughness and senses the shape of grooves that are bridged by the runner 2, because the latter has a far larger effective radius. Thus, the runner 2 acts as a sort of high pass filter.
In order to better collect the waviness of a surface F, there are also runner sensors having two runners.
A reference plane sensor 5 stands out due to the fact that it is fixedly connected with a reference plane E. This reference plane E mostly resides in the heading part 6 of the scanner 3. A measurement with a reference plane sensor 5 yields, in comparison to a runner sensor 1, more precise and more comprehensive measurement results, in which besides the surface roughness the shape and the waviness of the constructional element can also be collected.
A runner sensor 1 is used, for example, in cases where no exact conclusions on the waviness or the 3-dimensional shape are desired. An advantage of a runner sensor 1 is the simple manageability, because measurements can be performed quickly and without significant alignments.
In FIGS. 2A and 2B the scanning results of a runner sensor 1 (FIG. 2A) and a reference plane sensor 5 (FIG. 2B) are shown. The scanning result shown in FIG. 2B corresponds best to the reality, because a reference plane sensor 5 records the true course of the surface as far as possible unaltered. In particular, in the case of deep grooves or of isolated high peaks S1 and S2 (see FIG. 2B), the errors of the different runner sensors become noticeable. For example, the behavior in front of a peak S1 is characteristic for a runner sensor 1. The runner 2 reaches the peak S1 before the probe tip 4. Thereby, the whole scanner 3 is lifted and the probe pin 4 then projects further downward from the surrounding scanner casing. This is recorded in FIG. 2A in such a way (see range B1) as if the probe pin 4 would run apparently into a hollow of the surface F before reaching the peak S1. The same holds analogously when reaching the peak S2.
However, a runner sensor may also yield falsified results in other situations. For example, this is the case when the movement of the runner 2 superimposes constructively with the movement of the probe tip 4 and thus a too large output signal is provided, or when the movements extinguish themselves wholly or partially and thereby a too small signal is provided. These problems occur for example for periodic surfaces. Further details regarding the previously known roughness sensing systems can be taken for example from the following non-fiction book: “Rauheitsmessung: Theorie und Praxis” (English translation: “Roughness Measurement: Theory and Praxis”), by Raimund Volk, published by Beuth Verlag, 2005, ISBN 3410159185, 9783410159186.
For 3-dimensional constructional elements, it is often important to draw a conclusion on the local coincidence, which means, that for different constructional elements of a series the roughness has to be measured respectively at exactly the same position.
Beside the determination of the roughness, often the detection of the 3-dimensional shape, i.e., for example, the curvature of a surface (e.g., the convexity of a tooth of a gear wheel) or even the determination of the geometry, is also concerned.